In recent years, in terms of prevention of global warming and the like, a further reduction in power consumption in a motor of an air conditioner, main motor of an electric vehicle, and the like has been required. These motors are often used by being rotated at high speed. Accordingly, a non-oriented electrical steel sheet used for an iron core of a motor has been required to improve (reduce) a core loss in a frequency region of 400 Hz to 800 Hz higher than 50 Hz to 60 Hz being a commercial frequency. This is because the reduction in core loss reduces power consumption, thereby allowing an amount of energy consumption to be reduced.
Then, conventionally, as a technique to improve a core loss in a high frequency region, there has been employed a technique of increasing Si and Al contents to thereby increase electrical resistance. Ti is also contained in a raw material of Si and a raw material of Al, and when the Si and Al contents are increased, an amount of Ti to be inevitably mixed in a non-oriented electrical steel sheet is also increased.
In a treatment process of a non-oriented electrical steel sheet, or the like, Ti produces inclusions such as TiN, TiS and TiC, (which will be sometimes described as Ti inclusions, hereinafter), in the non-oriented electrical steel sheet. The Ti inclusions hinder the growth of crystal grains at the time of annealing of the non-oriented electrical steel sheet and suppress the improvement of a magnetic property. Particularly, a large number of Ti inclusions are likely to be finely precipitated in grain boundaries during stress relief annealing. Further, there is sometimes a case that a customer stamps a non-oriented electrical steel sheet shipped by a manufacturer, and thereafter performs stress relief annealing, for example, at 750° C. for two hours or so to thereby grow crystal grains. In the above case, even if Ti inclusions are extremely reduced at the time of shipment, but after the customer performs the stress relief annealing, a large number of Ti inclusions are to exist in the non-oriented electrical steel sheet. Thus, even though the stress relief annealing is performed, the growth of crystal grains is suppressed by a large number of Ti inclusions, so that it is difficult to sufficiently improve the magnetic property.
In order to reduce the Ti inclusions, it is conceivable to use a raw material having a reduced Ti content as the raw material of Si and the raw material of Al, but such a raw material is very expensive. Further, it is also conceivable to reduce N, S, and C contents in the non-oriented electrical steel sheet. It is technically possible to reduce the S and C contents by a vacuum degassing treatment or the like, but a prolonged treatment is required and productivity reduces. Further, a large amount of N is contained in the atmosphere, so that it is difficult to avoid N mixing in molten steel. Even though sealing of a refining vessel is enhanced, the manufacturing cost is only increased, so that it is difficult to sufficiently suppress the mixture of N.